Mounting for a glow discharge cathode



July 8, 1969 AK P. wALcH MOUNTING POR GLOW DISCHARGE CA'IHOD 4Filed Nov. 17, 1965 INVENTOR. ALLAN P. WALCH "w D v @2Q/MJD ATTOR Y QTL United States Patent() MOUNTING FOR A GLOW DISCHARGE CATHODE Allan P. Walch, Manchester, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Nov. 17, 1965, Ser. No. 508,201

Int. Cl. H01j 1/94 U.S. Cl. 313-238 6 Claims This invention relates to a cold cathode. More specilically, it relates to a glow discharge cathode for producing a beam of electrons.

Conventional means for producing electron beams involve the liberation of electrons from the surface of a heated cathode by thermionic emission. Recently, electron beams have been produced from an apertured, cold hollow cathode as a result of the impact of high energy electrons with gas molecules within a hollow chamber enclosed by the cathode. In glow discharge devices various operating modes may be encountered if one, for instance, varies the potential difference between the cathode and the anode and the gas density of the environment. One of these modes produces a well-defined electron beam which may predictably and 'advantageously be used to work materials. `Other modes such as the arc mode is a good source of electrons but this mode exhibits erratic behavior and does not produce a beam.

These apertured hollow cathodes have hollow chambers fabricated from a wire mesh or solid metal with a single aperture in one end. When the cathode is subjected to a high negative potential with respect to an anode and with the proper cathode geometry and pressure level in the hollow chamber, a well-defined pencil beam of high current density, high energy electrons emanates from the aperture. An example of the versatility of the conligurations possible with the aperture hollow cathode may be found in the copending application Ser. No. 417,339, filed Dec. 10, 1964, now abandoned, entitled, Annular Hollow Cathode Discharge Apparatus, by Fernand J. Ferreira and assigned to the same assignee.

The efficiency of glow discharge cathodes operating in the electron beam mode can be substantially improved by the use of a shield surrounding the cathode. These shields suppress the emission of electrons from those cathode surfaces which do not contribute to the main beam. The shields may be made of a conductor or an insulator material such as is disclosed in the copending aplication Ser. No. 506,237, filed Nov. 3, 1965, now abandoned, by Conrad M. Banas and Clyde O. Brown, now abandoned, entitled Insulator Shielded Cathode and assigned tothe same assignee.

Since all of the external surfaces of the cathode operate in the electron beam mode, a plasma which assumes substantially the anode potential surrounds the cathode. This plasma is detrimental to eicient operation since not all of it contributes electrons for the main beam and the energy supplied to maintain it is dissipated at the cathode. The incorporation of shielding to eliminate the glow discharge from some of the external surfaces has substantially improved the beam power efficiency of such cathodes.

The gap between the cathode and the shield must be adjusted to be less than the mean free path established by the gaseous environment to inhibit the formation of the plasma in the gap. But the gap can neither be too long in length since electrons and ions may be able to travel along tangential paths substantially greater than the mean free path. Consequently, the mechanical design between the cathode and the shield must take this factor into account and long path lengths must be especially avoided near the region Where the biasing lead is connected to the cathode and protrudes into the evacuated chamber.

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It is therefore an object of this invention to provide a shielded glow discharge cathode with minimum arcing features and supports.

This and other objects of this invention will become more readily apparent upon a review of the following description and the accompanying figure.

In the ligure, a side view of an annular large apertured hollow cathode supported by this invention is shown.

In the figure, 1 a hollow cathode of the types described in the copen'ding application Ser. No. 508,302 entitled, Large Apertured Hollow Cathode, filed Nov. 17, 1965, by F. J. Ferreira and assigned to the same assignee is located in a chamber of which only a portion 18 of one of its walls is shown and which is evacuated to the appropriate pressures necessary for establishing a glow discharge from the hollow cathode. The cathode 2 is an annular cavity defining an aperture 3 coaxial with a workpiece 4.

The workpiece 4 is connected to the positive side of a high voltage source 5 which in turn has its negative side connected to the cathode via the conductor 6. If the workpiece is a nonconductor any large conducting surface within the chamber may serve as an anode to which the positive source is connected. The cathode 2 is supported at several places around its perimeter with conductive pins 7. The pins 7 are conductive to avoid high electric eld stresses at the junction of the pins and the cathode. The conductor `6 is also connected to the cathode body 2 and provides additional support. The cathode 2 is surrounded by a similarly shaped shield 8 which may be a conductor or an insulator and is separated therefrom by a gap 9, the size of which is determined Eby the pressure in the chamber and the anode potential as well as the type of gas in the chamber. The gap 9 must be sufficiently small to inhibit the formation of a plasma therein. The shield 8 also has an aperture 10 coaxial with the aperture 3 of the cathode 2 but its radius is slightly larger than the aperture 3.

The conductive support 7 of the cathode 2 has a threaded section which mates into a threaded socket in the cathode 2 and is mounted in an insulated insert 12. The insert 12 has a two-part cavity 14 which is substantially coaxial with the support 7. The narrow portion of the cavity 14 snugly ts around the support 7 which can slide within it as a result of temperature expansions of the cathode 2.

Although the shield 8 improves the eiciency of the glow discharge cathode significantly, some power is still dissipated in the cathode so that it encounters a significant rise in temperature. The temperature expansions of the cathode may be absorbed by the slidable mountings of the support 7 and the conductor y6. The reduce excessive variations in the gap 9 at any one place a symmetrical mounting arrangement of three supporting points equally spaced about the cathode perimeter can be used.

The Wider portion of the cavity 14 is shaped to minimize the formation of a plasma within the region immediately adjacent the support 7. By holding the length 16 of the cavity 14 substantially equal or less than the mean free path length and the width 19 of the cavity 14 less than the mean free path, very little plasma is formed in the cavity and arcing is eliminated.

The conductor 6 mates with a cavity in the cathode 2 within which it can slideably move as a result of temperature expansions of the cathode and yet retain electrical contact with it. At the other end conductor 6 passes through an insulator, ceramic feedthrough 20* with a similar shape adjacent the gap 9 as that for the insert 12. The feedthrough 20 encloses the combination of the conductor inside a ceramic sleeve 22. The sleeve 22 is shorter than the feedthrough 20 in order to produce a cavity 24 having a length and width similar, respectively, to length 16 and width 19 for the cavity 14. The feedthrough 20 fits within a conducting bushing 26, having a threaded section at either end with a ange 27 in between and mounted to the outside of the wall of the shield 8. The other threaded portion of the bushing 17 receives a nut 28 having a countersunk threaded hole.

The feedthrough 20 is further provided with a brazedon flare 30. At the end of the feedthrough 2l)` a metal flange 32 is provided with a threaded section and a centrally located hole to be electrically connected to a high voltage cable 34. The conductor 6 fits into and is brazed into the hole of flange 32 to form a hermetic seal therewith.

The high voltage cable 34 is enclosed in a metal sheath 36 having a flanged end which is placed adjacent the flare 30. The fastening of the nut 28 then compresses the flare 30, the sheath 36 and the bushing 26 to form a hermetic seal preventing the leakage of gas into the evacuated chamber from the high voltage cable 34. The sheath 36 is brazed to the wall 18 of the evacuated chamber in which the assembly is located to form a hermetic seal therewith. An insulator sleeve 37 is slipped over the high voltage connection between the high voltage cable 34 and feedthrough 20 to prevent a discharge from the connection'to the metal sheath 36.

After evacuation to the proper pressure level and upon activation of the biasing voltage, the glow discharge mode is established and the annular beam emerges from the cavity enclosed by the cathode 2 towards the workpiece 4 which may then be worked by the beam.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

I claim:

1. In a shielded glow discharge cathode device for producing an electron beam comprising:

means for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode,

said cathode having an aperture and provided with a first recess,

a shield selectively spaced from and surrounding said cathode and having an opening opposite to and similarly shaped as the cathode aperture,

said shield having an insulator feedthrough provided with a passageway,

where said feedthrough is located opposite said cathode recess,

a conductive member hermetcally sealed to one end of said feedthrough and located inside said passageway and slideably contacting said cathode in said first recess.

2. A device as recited in claim 1 wherein said shield is further provided with a recess,

a second conductive member connected to said cathode and located to slideably mount within said shield recess.

3. A device as recited in claim 1 wherein the insulator feedthrough is further provided with a cavity coaxial with said first conductive member,

said cavity further having a width larger than said conductive member and shaped to suppress the formation of said glow discharge therein.

4. A device as recited in claim 3 wherein said shield is further provided with a recess,

a second conductive member connected to said cathode and located to slideably mount within said shield recess.

5. A device as recited in claim 4 where said first conductive member is enclosed by a first insulator sleeve having a length smaller than said feedthrough to form said feedthrough cavity adjacent the space between the shield and the cathode.

6. A device as recited in claim 5 where the shield recess is further provided with a cavity coaxial with said second conductive member and having a shape to suppress the formation of said glow discharge therein,

said cavity in communication with the spacing between said shield and said cathode.

No references cited.

JOHN W. HUCKERT, Primary Examiner.

R. F. POLISSACK, Assistant Examiner.

U.S. C1. X.R. 

1. IN A SHIELDED GLOW DISCHARGE CATHODE DEVICE FOR PRODUCING AN ELECTRON BEAM COMPRISING: MEANS FOR ESTABLISHING A GLOW DISCHARGE AND A CATHODE FALL FOR THE PRODUCTION OF AN ELECTRON BEAM FROM THE CATHODE, SAID CATHODE HAVING AN APERTURE AND PROVIDED WITH A FIRST RECESS, A SHIELD SELECTIVELY SPACED FROM AND SURROUNDING SAID CATHODE AND HAVING AN OPENING OPPOSITE TO AND SIMILARLY SHAPED AS THE CATHODE APERTURE, SAID SHIELD HAVING AN INSULATOR FEEDTHROUGH PROVIDED WITH A PASSAGEWAY. 